In osmotically pumped heat pipes, the pumping rate of solvent through a solvent permeable membrane is controlled by the concentration gradient between the solvent and a solute-solvent mixture on opposite sides of the membrane. In order to maintain high pumping rates, the concentration gradient must be maintained as high as possible. Otherwise, flow through the membrane will decrease with time as solvent, which is freshly pumped through the membrane from the solvent reservoir side thereof, forms a film on the solution side of the membrane. Although this freshly pumped solvent does mix with the solution, it does so slowly and, even then, dilutes the solution. The result is to decrease the high concentration gradient on either side of the membrane, and to decrease correspondingly the pumping rates. This problem has been overcome in the past by use of mechanical pumping of solute past the membrane surface to remove the freshly pumped solvent. This technique consumes power and reduces system reliability.